In a surprise move against a key US ally, Trump suggested Qatar -- home to the largest American airbase in the Middle East -- was funding extremism as he tacitly backed the diplomatic blockade of the emirate.

"So good to see the Saudi Arabia visit with the King and 50 countries already paying off," Trump said in a morning tweet, in reference to his trip to Riyadh last month.

"They said they would take a hard line on funding... extremism, and all reference was pointing to Qatar. Perhaps this will be the beginning of the end to the horror of terrorism!"

Trump's broadside came as Kuwait's ruler flew to Saudi Arabia in a bid to resolve the worst diplomatic crisis to hit the Arab world in years.

"This is the emptiest airport I have been in so far," said Katie, transiting in Qatar en route to Thailand. "It's exceptionally quiet, almost eerie."

The crisis will have wide-ranging consequences, not just for Qatar and its citizens but across the Middle East and for Western interests.

Qatar is home to the biggest US airbase in the Middle East, Al-Udeid, where some 10,000 military personnel are stationed. As the forward headquarters of US Central Command, it is seen as crucial in the US-led campaign against the Islamic State group.

A Pentagon spokesman said the crisis has had "no impact on our operations in Qatar or with regards to airspace permission around it".

Qatar has also been criticised for supporting Islamist rebels in Syria, and in 2013, the Afghan Taliban opened a Doha office.

Fellow Gulf states are also reported to have been angered by a huge ransom paid by Doha earlier this year to secure the release of a hunting party, which included members of the Qatari royal family, kidnapped in southern Iraq.

The ransom, which Iraqi officials said was in the "hundreds of millions of dollars", was believed to have been paid to militias with close ties to Tehran.